New dwarf planet hints at giant world far beyond Pluto

A surprise monster may be lurking in our solar system. A newly discovered dwarf planet has grabbed the crown as the most distant known object in our solar system – and its orbit hints at a giant, unseen rocky world, 10 times the mass of Earth and orbiting far beyond Pluto.

The dwarf planet, for now dubbed 2012 VP113 because it was spotted in images taken in November 2012 – is an interesting discovery in itself. Scott Sheppard of the Carnegie Institution for Science in Washington DC and his colleagues found that it is a lump of rock and ice 450 kilometres wide and lies at 80 astronomical units from the sun at its closest approach (1 AU is Earth's distance from the sun).

That's twice as far as the most famous dwarf planet, Pluto, which is 2340 kilometres wide and also beats the previous record holder, a 1000-kilometre-wide planetoid called Sedna, discovered in 2003, with a closest approach of 76 AU out.

Objects orbiting this far from the sun, in the "inner Oort cloud", are useful to probe the early solar system. That's because they lie too far away to be perturbed by the gas planets, but too close to the sun to be affected by the gravity of other stars in our galaxy – so their orbits and behaviour are thought to be almost unchanged since they first formed. "Once we find more objects in this region, we'll be able to start to strongly constrain the possible formation scenarios," says Sheppard.

Distant puppeteer

The discovery of 2012 VP113 instantly doubles the number of objects known in this part of space, from 1 to 2 – and the team estimates that many more exist in this region and are just waiting to be discovered. "We thought Pluto was unique for over 70 years, but we now know that it shares its orbit with thousands of other objects," says Sheppard. "Sedna was unique for about 10 years but it's now clear that Sedna and 2012 VP113 are just the tip of the iceberg."

Intriguingly, Sheppard's team also found a strange alignment when they looked at the orbits of 2012 VP113, Sedna and 10 other objects that lie closer to the sun. "It was a big surprise to us," he says.

One explanation for the alignment could be the tug of a rocky planet that is 10 times the mass of Earth that orbits the sun at 250 AU, the team calculate. That world would be cold and faint – and would push and pull at the closer objects like a distant but powerful puppeteer.

However, WISE was looking for the tell-tale warmth of gas giants – a rocky "super-Earth", like the one Sheppard's team suggest, would be too cold for the telescope to pick up. "This is too faint for WISE," says Ned Wright, the space telescope's principal investigator. Even if the planet has a small internal heat source – and absorbs some sunlight, it would still not generate enough heat to register, he adds.

Kepler is designed to spot tell-tale dips in the brightness of alien stars as planets pass in front of them. "No way would Kepler find this planet," says William Welsh of San Diego State University, a member of the Kepler team.

For that reason, Mike Brown of the California Institute of Technology, a co-discoverer of Sedna, isn't holding out hope of a giant planet just yet. "It is possible that some undiscovered large object out there is doing this, but there are likely many other explanations, too, most of them sadly more mundane."

Journal reference: Nature, DOI: 10.1038/nature13156

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Three images, showing dwarf planet 2012 VP113 in red, then green, then blue, were combined to reveal its path across the night sky (Image: Scott S. Sheppard, Carnegie Institution for Science)